Field of the Invention
Embodiments of the invention generally relate to enhanced yield-related traits in plants. In particular, the invention provides plants that have been genetically engineered to down-regulate expression or reduce the activity of BPM proteins, resulting in enhanced yield-related traits including without limitation enhanced seed oil production, as well as products made by or from the plants.
Background of the Invention
Under field conditions, plant performance, for example in terms of growth, development, biomass accumulation and seed generation, depends on a plant's tolerance and acclimation ability to numerous environmental conditions, changes and stresses. There has always been a need for improving plant traits in crop cultivation. Breeding strategies foster crop properties to withstand biotic and abiotic stresses, to improve nutrient use efficiency and to alter other intrinsic crop specific yield parameters.
Plants are sessile organisms and consequently need to cope with various environmental stresses. Biotic stresses such as plant pests and pathogens on the one hand, and abiotic environmental stresses on the other hand are major limiting factors for plant growth and productivity (Boyer, 1982; Bohnert et al., 1995), thereby limiting plant cultivation and geographical distribution. Plants exposed to different stresses typically have low yields of plant material, like seeds, fruit or other produces. Crop losses and crop yield losses caused by abiotic and biotic stresses represent a significant economic and political factor and contribute to food shortages, particularly in many underdeveloped countries.
Conventional means for crop and horticultural improvements today utilize selective breeding techniques to identify plants with desirable characteristics. Advances in molecular biology have allowed for the production of transgenic plants with enhanced yield-related traits. Various yield-related traits in plants are important to many industries worldwide. In particular, plant seed oils are an important source of calories for human nutrition, as feedstocks for non-food uses such as soaps and polymers, and can serve as a high-energy biofuel. World production from oilseed crops in 2011 reached a value near US$120 billion with plant oil consumption expected to double by 2040 (Bates et al., 2013). As a result, methods for increasing seed oil biosynthesis have been an important research topic. However, previous attempts to modulate the transcription levels of factors critical for seed oil biosynthesis, such as WRINKLED1 (WRI1), resulted in relatively low increases in seed oil content (Liu et al., 2010; Shen et al., 2010; Pouvreau et al., 2011).
Effective regulatory mechanisms to time and control developmental and physiological processes in response to environmental cues are of utmost importance to plants due to their sessile life style. A mechanism that allows plants to quickly and flexibly respond is the ubiquitin (UBQ) proteasome pathway (Hua and Vierstra, 2011). It is highly conserved among eukaryotes and requires the concerted activities of an E1 UBQ activating enzyme, a UBQ conjugating enzyme E2, and an E3 UBQ ligase. While E1 and E2 activate the UBQ to modify target substrates, the E3 ligase binds the E2 and a substrate protein to facilitate transfer of the UBQ moiety. Upon building up a UBQ chain on the substrate, the ubiquitylated protein is marked for degradation via the 26S proteasome (Hua and Vierstra, 2011).
CUL3-based RING E3 ligases (CRL3) have been described only recently and mainly with respect to their basic architecture (Figueroa et al., 2005; Gingerich et al., 2005; Weber et al., 2005; Gingerich et al., 2007). They are composed of a cullin 3 protein, as the scaffolding subunit, that binds in its C-terminal region the RING-finger protein RBX1, while its N-terminal part is recognized by proteins containing a BTB/POZ (Broad complex, Tramtrack, Bric-a-brac/Pox virus and Zinc finger) fold (Figueroa et al., 2005; Weber et al., 2005). BTB/POZ proteins comprise a diverse group of proteins within Arabidopsis and rice, containing 80 and 149 members, respectively (Gingerich et al., 2007). They have been divided into 12 subgroups based on their secondary domains (Gingerich et al., 2007). While the BTB/POZ fold is required for assembly with the cullin and to interact with other BTB/POZ proteins, the secondary domain may function as an adaptor to allow binding of a substrate and delivery to the CRL3 core for ubiquitylation.
Based on its role as the central scaffolding subunit that assembles with potentially many BTB/POZ proteins, it is not surprising that the loss of CUL3 causes an embryo lethal phenotype. Reduced amounts of functional cullin 3 protein affects red light and ethylene signaling and impacts plant development (Dieterle et al., 2005; Thomann et al., 2009).
One BTB/POZ subfamily is the BPM (BTB/POZ-MATH) family that contains a BTB/POZ fold in their C-terminal region, and a MATH (Meprin and TRAF [tumor necrosis factor receptor associated factor] homolog) domain located within the first 200 amino acids of their N-terminal region. BPM proteins are known in the art and may also be referred to as MATH-BTB/POZ proteins. The family comprises six members in Arabidopsis, all of which have molecular weights between 40-50 kDa (Weber et al., 2005). A recent study of Brassica rapa provided a phylogenetic analysis of select BPM proteins, but there has yet to be any functional characterization of these genes in the Brassica species (Zhao et al., 2013). In Zea mays, it was found that the loss of a BPM protein resulted in defects in female gametophyte development (Juranić et al., 2012). However, there has been no study linking the downregulation of BPM proteins to enhanced yield-related traits.